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2.25 Magnet
Stick �em up with our fun and functional magnets. Holds refrigerator notes, photos, dress up a school locker, room or workspace (Also Available in 10 & 100 Packs)... |
Personalized Fridge Rectangle Magnet
Add instant style to lockers, dorm rooms, workspace or the fridge while keeping your notes and reminders in view with an attention(Also Available in 10 & 100 Packs)... |
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Examples of materials with very high permeability include iron and steel.
Liquid oxygen is an example of something with a low permeability, and it
is only weakly attracted to a magnetic field. Water has such a low
permeability that it is actually slightly repelled by magnetic fields.
Everything has a measurable permeability: people, gases, and even the
vacuum of outer space.
The SI unit of magnetic field strength is the tesla, and the SI unit of total
magnetic flux is the weber. 1 weber = 1 tesla flowing through 1 square
meter, and is a very large amount of magnetic flux.
Physical origin of magnetism
Permanent magnets
Normal pieces of matter are composed of particles such as protons,
neutrons, and electrons; and all of these have the fundamental property
of quantum mechanical spin. Spin gives each one of these particles an
associated magnetic field. Because of this, and the fact that the average
microscopic piece of matter contains huge numbers of these particles, it
would be expected that all matter would be magnetic. Even antimatter
would have magnetic characteristics. However, everyday experience
shows that this is not the case.
Within each atom and molecule, the spin of each of these particles is
highly ordered as a result of the Pauli Exclusion Principle. However, there
is no long-range ordering of these spins between atoms and molecules.
Without long-range ordering, there is no net magnetic field because the
magnetic moment of each one of the particles is canceled by the magnetic
moment of other particles.
Permanent magnets are special in that long-range ordering does exist.
The highest degree of ordering exists within magnetic domains. These
domains can be likened to microscopic neighborhoods in which there is a
strong reinforcing interaction between particles, and as a result, a great
deal of order. The greater the degree of ordering within and between
domains, the greater the resulting field will be.
Long-range ordering (and the resulting strong net magnetic field) is one
of the hallmarks of a ferromagnetic material.
Electronic generation of magnetism
Electrons play the primary role in generating a magnetic field. Within an
atom, electrons can exist either individually or in pairs within any given
orbital. When they are paired, the individuals in that pair always have
opposite spin—one up, one down. The fact that the spins have opposite
orientation means that the two cancel one another. If all electrons are
paired, no net magnetic field will be generated.
In some atoms, there are electrons that are unpaired. All magnets have
unpaired electrons, but not all atoms with unpaired electrons are
ferromagnetic. In order for the material to become ferromagnetic, not
only must there be unpaired electrons present, but those unpaired
electrons must interact with one another over long ranges such that they
are all oriented in the same direction. The specific electron configuration
of the atoms (as well as the distance between atoms) is what leads to
this long-range ordering. Electrons exist in a lower energy state if they
share the same orientation.
Electromagnets
An electromagnet, in its simplest form, is a wire that has been coiled into
one or more loops. This coil is known as a solenoid. When electric current
flows along the coil, a magnetic field is generated around the coil. The
orientation of this field can be determined via the right hand rule. The
strength of the field is influenced by several factors. The number of loops
determines the surface area of interaction, the amount of current
determines the amount of activity, and the material in the core
determines electrical resistance. The more loops of wire and the greater
the current, the stronger the field will be.
If the coil of wire is empty in the center, it will tend to generate a very
weak field. Different ferromagnetic or paramagnetic items can be placed in
the center of the core with the effect of magnifying the magnetic field, for
example an iron nail. In addition, soft iron is commonly used for this
purpose. The addition of these types of materials can result in a several
hundred- to thousand-fold increase of field strength.
At distances which are large compared to the magnet's dimension, the
observed magnetic field obeys an inverse cube law. This means that the
field strength is inversely proportional to the third power of the distance
from the magnet.
In the case of an electromagnet in contact with a flat metal plate, the
force needed to separate the two will be greatest if the two surfaces are
machined as flat as possible. The flatter the surfaces, the more points of
contact between them, and the smaller the magnetic circuit's reluctance
to the magnetic field.
Electromagnets find uses in many places, ranging from particle
accelerators, to electric motors, to junkyard cranes, to magnetic
resonance imaging machines. There are also specialized applications that
involve more than a simple magnetic dipole, such as the quadrupole
magnets used to focus particle beams.
If enough electric current is passed through the coil of an electromagnet,
the magnetic force between neighboring loops of wire can cause the
electromagnet to be crushed by its own magnetic field.
Characteristics of magnets
Permanent magnets and dipoles
All magnets have at least two poles: that is, all magnets have at least one
north pole and at least one south pole. The poles are not a pair of things
on or inside the magnet. They are a concept used to discuss and
describe magnets. In the image at the top of this page, the poles look
like specific locations, because the highest surface intensity of the field
occurs at the poles, but this does not mean that they are specific
locations.
To understand the concept of pole, it can be imagined that a row of
people who are all facing the same direction and standing in line. While
there is a "face" end of the line and a "back" end of the line, there is no
one place where all of the faces are and all of the backs are. The person
at the front of the face end has a back; and the person at the back end
has a face. If the line is divided into two shorter lines, each one of the
shorter lines still has a face end and a back end. Even if the line is pulled
completely apart so that there are just individuals standing around, each
one of the individuals still has a face and a back. This can continue
without end.
The same holds true with magnets. There is not one place where all of
the north or south poles are. If a magnet is divided in two, two magnets
will result and both magnets will have a north and a south pole. Those
smaller magnets can then be divided, and all of the resulting pieces will
have both a north and south pole. In most instances, if the material
continues to be broken into smaller and smaller pieces there will be a
point where the pieces are too small to retain a net magnetic field. They
won't become individual north or south poles though; instead, they will
just lose the ability to maintain a net field. Some materials, however, can
be divided down to the molecular level and still maintain a net field with
both a north and a south pole. There are theories involving the possibility
of north and south magnetic monopoles, but no magnetic monopole has
ever been found.
A standard naming system for the poles of magnets is important.
Historically, the terms north and south reflect awareness of the
relationship between magnets and the earth's magnetic field. A freely
suspended magnet will eventually orient itself north-to-south, because of
its attraction to the north and south magnetic poles of the earth. The
end of a magnet that points toward the Earth's geographic North Pole is
labeled as the north pole of the magnet; correspondingly, the end that
points south is the south pole of the magnet.
The Earth's current geographic north is thus actually its magnetic south.
Confounding the situation further, magnetised rocks on the ocean floor
show that the Earth's magnetic field has reversed itself in the past, so
this system of naming is likely to be backward at some time in the future.
Fortunately, by using an electromagnet and the right hand rule, the
orientation of the field of a magnet can be defined without reference to
the Earth's geomagnetic field.
To avoid the confusion between geographic and magnetic north and
south poles, the terms positive and negative are sometimes used for the
poles of a magnet. The positive pole is that which seeks geographical
north.
Common uses for magnets and electromagnets
• Magnetic recording media: Common VHS tapes contain a reel of
magnetic tape. The information that makes up the video and sound is
encoded on the magnetic coating on the tape. Common audio cassettes
also rely on magnetic tape. Similarly, in computers, floppy disks and hard
disks record data on a thin magnetic coating.
• Credit, debit, and ATM cards: All of these cards have a magnetic
strip on one of their sides. This strip contains the necessary information
to contact an individual's financial institution and connect with their
account(s).
• Common televisions and computer monitors: The majority of TV's
and computer screens rely in part on an electromagnet to generate an
image--see the article on cathode ray tubes for more information. Plasma
screens and LCDs rely on different technology entirely.
• Loudspeakers and microphones: Loudspeakers actually rely on a
combination of a permanent magnet and an electromagnet. A speaker is
fundamentally a device to convert electric energy (the signal) into
mechanical energy (the sound). The electromagnet carries the signal,
which generates a changing magnetic field that pushes and pulls on the
field generated by the permanent magnet. This pushing and pulling
moves the cone, which creates sound. Not all speakers rely on this
technology, but the vast majority do. Standard microphones are based
upon the same concept, but run in reverse. A microphone has a cone or
membrane attached to a coil of wire. The coil rests inside a specially
shaped magnet. When sound vibrates the membrane, the coil is vibrated
as well. As the coil moves through the magnetic field, a voltage is
generated in the coil (see Lenz's Law). This voltage in the wire is now an
electric signal that is representative of the original sound.
Magnetic hand separator for heavy minerals
• Electric motors and generators: Some electric motors (much like
loudspeakers) rely upon a combination of an electromagnet and a
permanent magnet, and much like loudspeakers, they convert electric
energy into mechanical energy. A generator is the reverse: it converts
mechanical energy into electric energy.
• Transformers: Transformers are devices that transfer electric
energy between two windings that are electrically isolated but are linked
magnetically.
• Chucks: Chucks are used in the metalworking field to hold objects.
If these objects can be held securely with a magnet then a permanent or
electromagnetic chuck may be used. Magnets are also used in other types
of fastening devices, such as the magnetic base, the magnetic clamp and
the refrigerator magnet.
• Magic: Naturally magnetic Lodestones as well as iron magnets are
used in conjunction with fine iron grains (called "magnetic sand") in the
practice of the African-American folk magic known as hoodoo. The stones
are symbolically linked to people's names and ritually sprinkled with
magnetic sand to reveal the magnetic field. One stone may be utilized to
bring desired things to a person; a pair of stones may be manipulated to
bring two people closer together in love.
• Art: 30 millimetre or thicker vinyl magnet sheets may attached to
paintings, photographs, and other ornamental articles, allowing them to
be stuck to refrigerators and other metal surfaces.
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